This invention relates to fuel cells and, in particular, to a fuel processing system for use with such fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Current fuel cell technology requires clean fuel gas composed of hydrogen or a mixture of hydrogen and carbon monoxide, which can be generated from hydrocarbon-containing feedstocks such as natural gas, petroleum-based liquids or coal through a reforming process. Most hydrocarbon-containing feedstocks contain sulfur, which causes reforming and anode catalyst poisoning and is known to significantly diminish the performance of fuel cell anodes and reforming catalysts. Therefore, as part of the reforming process, sulfur and sulfur-containing compounds have to be removed from the fuel gas to a part per billion level before the fuel gas enters the fuel cell.
The present state-of-the-art uses a combination of a hydrodesulfurization reaction with a zinc oxide adsorption bed in order to remove sulfur-containing compounds from the fuel gas. This method is generally used to remove sulfur-containing compounds, particularly mercaptan odorant compounds, from natural gas fuel. Hydrodesulfurization reaction is accomplished by reacting sulfur-containing compounds in the fuel with recycled hydrogen to produce hydrogen sulfide. During the hydrodesulfurization reaction, the fuel is passed over a catalyst where sulfur-containing compounds in the fuel react with hydrogen to produce hydrogen sulfide, or, alternatively, sulfur-containing compounds in the fuel can be reacted with hydrogen at temperatures above 570 degrees Fahrenheit (hot gas desulfurization).
In a conventional hot gas desulfurization system, fuel gas is mixed with recycled reformed gas prior to entering a heat exchanger, where it is heated to 570–750 degrees Fahrenheit and undergoes a reaction with the hydrogen in the recycled reformed gas to produce hydrogen sulfide (H2S). The fuel gas is then delivered to a desulfurizer where the H2S is removed from the fuel gas by adsorption in a zinc oxide bed. The resulting desulfurized fuel gas may then be delivered to the fuel processor of the fuel cell.
In the hydrodesulfurization system using a catalyst, the catalyst is used to react the sulfur-containing compounds in the fuel with the recycled hydrogen to produce hydrogen sulfide. In such a system, the fuel gas and the recycled reformed gas are first mixed and heated in the preheater vessel, after which the gas mixture is delivered to a hydrodesulfurization section. This section includes a hydrodesulfurization catalyst and a zinc oxide adsorbent in a single vessel. When the fuel gas mixture enters the hydrodesulfurization section, the sulfur-containing compounds are irreversibly converted to hydrogen sulfide (H2S) by the hydrodesulfurization reaction over the hydrodesulfurization catalyst, and the resulting H2S is adsorbed by the zinc oxide prior to exiting the hydrodesulfurization system.
Conventional desulfurization systems have also used multiple adsorbent beds to remove sulfur from fuel. For example, U.S. Pat. No. 3,620,969 teaches the use of two beds of crystalline zeolitic molecular sieve material piped so that when one bed is on the adsorption stroke, the other bed is being regenerated by purging and cooldown. In addition, U.S. Pat. No. 5,720,797 describes a pressure swing adsorption-desorption process for recovering sulfur hexafluoride from a gas stream using zeolites, activated carbons or silicates as adsorbents.
As can be appreciated from the above, the conventional hydrodesulfurization systems require high temperature and a hydrogen recycling system which supplies sufficient hydrogen concentration to convert sulfur-containing compounds to hydrogen sulfide. Moreover, depending on the hot desulfurizer operating temperature, an additional heat exchanger may be required to heat the gas to the required temperature.
With respect to the conventional activated carbon adsorbent systems, activated carbon adsorbents are selective to remove only certain types of sulfur-containing compounds and are not able to trap all organic and inorganic sulfur-containing compounds that are present in hydrocarbon-containing feedstocks. Particularly, most of the activated carbon adsorbents are unable to trap certain low molecular weight organic compounds such as dimethyl sulfide (DMS) and ethyl methyl sulfide (EMS).
In addition, the capacities of conventional adsorbent bed systems are relatively low to meet the size constraints of fuel cell systems, having a life of approximately 2 to 3 months. As a result, conventional systems are not very cost effective. Furthermore, conventional systems using multiple beds use the same adsorbent in each of the beds for continuous operation and thus are not able to remove all types of sulfur-containing compounds.
It is therefore an object of the present invention to provide a fuel processing system for fuel cells which is better able to remove all types of sulfur-containing compounds.
It is a further object of the invention to provide a high-capacity fuel processing system for fuel cells which has an increased life and is cost effective.